Two basic direction finding (DF) techniques have been widely used in the prior art to measure the angular coordinates of incoming aircraft. Both techniques employ two or more antennas to receive an electromagnetic signal transmitted by the aircraft or otherwise propagating from the aircraft's location. The signals received by the different antennas are then compared, whereupon the angular position of the aircraft is computed by a processor.
The first of these techniques, designated the Amplitude Comparison system, compares only the relative amplitude of the signals received by the different antennas. This approach is the least costly but also the least accurate one. Basically, a pair of antennas on an aircraft carder or ground location having beams that overlap in space are employed to simultaneously but independently receive the aircraft's signal, as shown in FIG. 1. The shown antennas A1 and A2, typically spiral antennas, have apertures squinted off the boresight axis (the x axis) at angles +.theta.s and -.theta.s, respectively. An incoming aircraft 10 approaching at an angle .theta. from the xz plane, transmits an RF signal that is received by the antennas A1 and A2. The shown orientation of the antennas A1 and A2 results in the antenna patterns G.sub.1 (.theta.) and G.sub.2 (.theta.) depicted in FIG. 2. These antenna patterns are typically broad with 3 dB beamwidths generally greater than 60.degree.. As such, a mathematical function can easily be derived which simulates these patterns. The measured amplitude (electromagnetic field strength) of the signals received by the antennas A1 and A2 can then readily be compared to one another to determine the azimuth angle .theta. of the aircraft relative to the ground location of the antennas. This is basically accomplished by means of a receiver 12, an amplitude comparison circuit 14 that converts the amplitude difference between the two signals to a digital word, a frequency detector 17 to detect the signal frequency and provide a digital word representing the same, and a processor 16. Stored in a memory within the processor 16 are the mathematical functions modelling the antenna patterns G.sub.1 (.theta.) and G.sub.2 (.theta.) as a function of frequency. The processor 16 then utilizes the digital word information as variables in the equations to compute the actual angle of arrival .theta..
While the Amplitude Comparison system just described is relatively simple and cost effective, its accuracy is typically on the order of 20 degrees which is rather poor. Accuracy is basically limited as a result of the inability to accurately measure small differences in amplitude between the received signals and due to inaccuracies in the mathematical models for the antenna patterns. Even if generalized ROM look up tables are used by the processor 16 for more precise antenna pattern information based on typical measured patterns at the factory, the variation in the actual patterns from antenna to antenna due to manufacturing tolerances will still cause significant inaccuracies.
The more precise, albeit complex prior art direction finding approach is known as phase interferometry or phase comparison. This technique is illustrated in FIG. 3, where antennas 20, 22 separated by a distance "d" independently receive the transmitted RF signal from the aircraft 10. With this approach, the planar apertures of the antennas 20, 22 lie in the same plane rather than being squinted away from one another. To determine azimuth positions, the antennas 20, 22 would be positioned on the y axis (of FIG. 1); to determine elevation angles, they would lie on the z axis. For the azimuth case, a plane wave propagating from an aircraft arriving at an angle .theta. from boresight (the x axis) is received by each of the two antennas 20 and 22. The phase difference .DELTA..phi. between the signals received by the two antennas is expressed as EQU .DELTA..phi.=(2.pi.d sin .theta.)/.lambda. (1)
where .lambda. is the wavelength of the signal propagating from the unknown aircraft location. This is illustrated geometrically by drawing from antenna 22, a line 25 representing the phase front of the incident plane wave. The plane wave travels an extra distance 1=d sin .theta. to reach antenna 20 as compared to antenna 22--thus the phase of the signal received by antenna 20 lags accordingly. The phase of the two received signals are compared by a phase comparator 23 and then frequency detected, with the results supplied to a processor 27 where the azimuth angle .theta. of the aircraft is readily computed from eqn. (1). By employing a third antenna (not shown) positioned along the z axis and having an aperture facing a direction parallel to the x axis, the elevation angle of the aircraft can similarly be computed by virtue of the phase relationship of the signal received by the third antenna with respect to that of either antenna 20 or 22, whichever is directly below the third antenna.
The primary drawback of the phase interferometer approach is that more than one angular position of the aircraft can produce the same phase relationship between the signals received by the two antennas. Consequently, ambiguities in angular position will result with the two antenna approach. The ambiguity problem can be solved by employing one or more additional antennas or pairs of antennas with different baseline spacings between these additional antennas. Ambiguities are then resolved by comparing electrical phase between several pairs of antennas. Once the ambiguities are eliminated, the angle of arrival accuracy of the phase interferometry system is high--better than 0.5 degree accuracy has been reported. However, finding adequate installation locations for the extra antennas renders this type of system impractical for small aircraft platforms.
As a compromise between the simple but inaccurate amplitude comparison system and the highly accurate but space inefficient phase interferometer system, a hybrid amplitude/phase comparison system was developed by ITT Corporation, the assignee herein. This system utilizes a pair of squinted antennas as in the amplitude comparison approach of FIG. 1. Using only this antenna pair, both a phase and an amplitude comparison between the two received signals are performed. The amplitude comparison is used to resolve the ambiguities in the arrival angle computed by virtue of the phase comparison. This hybrid approach results in accuracies on the order of 1-5 degrees.
Accordingly, it would be desirable to further improve the angle of arrival accuracy of such a hybrid amplitude/phase comparison system without the need for additional antenna space.
It is therefore an object of the present invention to provide a direction finding system employing the hybrid amplitude/phase comparison technique, with improved angle of arrival accuracy.